C3 Carbon Reduction Cycle

Abstract

The C3 carbon reduction cycle is the primary pathway of carbon fixation in all photosynthetic organisms, reducing carbon dioxide
from the atmosphere to form carbohydrates, and in higher plants, it takes place in the chloroplast stroma. The carboxylation,
reduction and regeneration phases of the Calvin–Benson cycle require several enzymes whose properties have been the focus
of intense study. The intermediates of the Calvin–Benson cycle serve as links with several other pathways, and photosynthetically
fixed carbon is exchanged among the pathways. Control of Rubisco involves several regulation mechanisms, and some are similar
to those of other enzymes of the Calvin–Benson pathway, such as thioredoxin redox control as well as multiprotein complexes.
All of these enzymes and control mechanisms, as well as plant canopy structure, are being studied with the aim of improving
photosynthesis for increased plant production to feed a growing population.

Key Concepts

Rubisco is the key carbon-fixing enzyme in photosynthesis. Its activity is inefficient for several reasons, including the
competition between its carboxylase and oxygenase activities, its kinetic properties, affinity for substrate, and the temperature
sensitivity of Rubisco activase.

The oxygenase activity of Rubisco causes photorespiration, which results in a loss of fixed carbon, especially under high
temperatures. Other plants and some bacteria have carbon concentrating mechanisms or photorespiratory CO2 trapping mechanisms
that allow for greater efficiency of carbon fixation. These provide opportunities for engineering of more efficient plant
production.

The C3 cycle produces carbon compounds that are used in plant growth and development, and so is linked to several other pathways.
The result is flexibility in responding to environmental changes and plant needs, but also interesting challenges in engineering
photosynthesis apparatus and Calvin-Benson cycle enzymes.

Activities of several of the enzymes of the C3 cycle are regulated by light, which allows coordination of the pathway as a whole. The light regulation through redox processes
also allows coordination with the electron transport chain. Light also has a central role in control of gene expression for
enzymes, along with other environmental factors.

The need to increase food production has stimulated research on photosynthetic and Calvin-Benson cycle limitations. Modelling
based strategies have allowed analysis of these limitations for potential areas of improvement.

Krapp A, Hofman B, Schafer C and Stitt M (1993) Regulation of the expression of rbcS and other photosynthetic genes by carbohydrates: a mechanism for the ‘sink’ regulation of photosynthesis? The Plant Journal 3: 817–828.